1. Field of the Invention
The present invention relates to a radiation imaging apparatus that irradiates a subject with a radioactive ray emitted from a radiation generation unit to capture a radiographic image based on the radioactive ray that has penetrated through the subject, and also relates to a method for driving the radiation imaging apparatus.
2. Description of the Related Art
There is a conventional X-ray imaging apparatus that can capture an image of a subject with an X-ray (i.e., one of various kinds of radioactive rays), so that the acquired X-ray image can be used for a medical care. In this case, the X-ray imaging apparatus includes an X-ray generation unit that can emit an X-ray toward the subject. The X-ray imaging apparatus acquires an X-ray image based on the X-ray having penetrated through the subject. Users can observe an internal state of the subject based on the acquired X-ray image. The X-ray imaging apparatus may also be referred to as an “X-ray fluoroscopic apparatus.” If the subject is a human body, the word “observation” may be replaced with “diagnosis” or “examination.”
The X-ray imaging apparatus further includes an X-ray sensor unit that can detect the X-ray having penetrated through the subject to acquire the X-ray image. The X-ray sensor unit is generally configured to detect an X-ray and generate an electric signal according to the intensity of the detected X-ray. Namely, the X-ray sensor unit is a device capable of acquiring an X-ray image based on the detected X-ray. An example of the X-ray sensor unit is an image intensifier or a flat panel detector (FPD).
To prevent a subject from being exposed to an excessive amount of X-ray that can be emitted from the X-ray generation unit, it is desired to reduce the irradiation X-dose that may be absorbed by the subject. However, if the amount of the irradiation X-dose that may be absorbed by the subject is small, the amount of the X-ray that can penetrate through the subject is small. As a result, the X-ray dose that can be received by the X-ray sensor unit is small. The acquired X-ray image may be inappropriate to observe the subject.
As described above, when the X-ray imaging apparatus captures an image of a subject, it is important to satisfy both an objective to “reduce the amount of X-ray dose that may be absorbed by the subject” and another objective to “acquire an X-ray image that is suitable for observation.”
Therefore, the X-ray imaging apparatus includes an X-ray irradiation condition determination unit that can automatically determine optimum X-ray irradiation conditions for an X-ray that can be emitted from the X-ray generation unit. The X-ray irradiation conditions, for example, include a tube voltage and a tube current of the X-ray generation unit and an irradiation time of the X-ray. The tube voltage is a voltage that may be applied to an X-ray tube of the X-ray generation unit that can generate an X-ray. The tube voltage relates to a spectrum of the X-ray. The tube current is current flowing through the X-ray tube of the X-ray generation unit. The tube current relates to the intensity of the X-ray. The irradiation time is a period of time during which the X-ray can be generated by an amount determined based on the tube voltage and the tube current.
In this case, if the subject that may be observed is always constant, it may be desired to continuously use the same X-ray irradiation conditions which have been once determined as optimum X-ray irradiation conditions. On the other hand, if the subject that may be observed is variable, or if the subject that may be observed is a moving subject, it is required to set optimum X-ray irradiation conditions in respective cases because the optimum X-ray irradiation conditions are variable depending on each subject.
In the conventional X-ray imaging apparatus, it is presumed that the X-ray sensor unit is an image intensifier when the X-ray irradiation condition determination unit determines the X-ray irradiation conditions. In this case, the image intensifier has a very narrow dynamic range (i.e., normal operation range). Therefore, in determining the X-ray irradiation conditions, it is generally required to prevent the X-ray dose that may enter the image intensifier from exceeding the dynamic range.
If the X-ray dose that may enter the image intensifier exceeds the dynamic range, it may be difficult to “acquire an X-ray image that is suitable for observation.” More specifically, in this case, halation (i.e., overexposure) may occur on the acquired X-ray image. If the X-ray dose that may enter the image intensifier is below the dynamic range, it may be difficult to “acquire an X-ray image that is suitable for observation” because of underexposure on the X-ray image.
A technique capable of solving the above-described problem is, for example, discussed in Japanese Patent Application Laid-Open No. 8-66389 or in Japanese Patent Application Laid-Open No. 2002-14059. The discussed technique determines X-ray irradiation conditions so as to set a predetermined value relating to the X-ray image in a very narrow normal operation range (i.e., dynamic range) of an image intensifier and then determines the X-ray irradiation conditions so as to reduce the X-ray exposure amount.
As described above, when the X-ray sensor unit is an image intensifier, it is generally very difficult to adequately satisfy both the purpose of “reducing the amount of X-ray that may be absorbed by the subject” and another purpose of “acquiring an X-ray image that is suitable for observation.”
However, the X-ray sensor unit that may be widely used is the FPD. Compared to the image intensifier, the FPD has a very wide dynamic range. Therefore, it is unnecessary to set the X-ray irradiation conditions in a very narrow range.
A conventional X-ray imaging apparatus may have the following configuration. FIG. 18 illustrates an example of the configuration of a conventional X-ray imaging apparatus 1800. The conventional X-ray imaging apparatus 1800, as illustrated in FIG. 18, includes an X-ray generation unit 1801, an X-ray sensor unit 1802, an X-ray irradiation condition determination unit 1803, and a display unit 1804. In this case, the X-ray sensor unit 1802 may be an image intensifier. The subject 1000 may be disposed between the X-ray generation unit 1801 and the X-ray sensor unit 1802.
In the conventional X-ray imaging apparatus 1800, the X-ray irradiation condition determination unit 1803 determines X-ray irradiation conditions so as to reduce the X-ray that may be absorbed by the subject 1000 considering a very narrow dynamic range of the X-ray sensor unit 1802, as described above.
The X-ray generation unit 1801 emits an X-ray (i.e., irradiation X-ray) 1801a toward the subject 1000 based on the X-ray irradiation conditions determined by the X-ray irradiation condition determination unit 1803. A transmission X-ray 1801b, i.e., part of the irradiation X-ray 1801a that has not been absorbed by the subject 1000 and has penetrated through the subject 1000, may enter the X-ray sensor unit 1802. The X-ray sensor unit 1802 generates an X-ray image “A” based on the transmission X-ray 1801b. The display unit 1804 displays the X-ray image “A.”
However, in this case, the X-ray image “A” is an image that has been obtained so as to satisfy the requirement of a very narrow dynamic range of the X-ray sensor unit 1802. Therefore, the X-ray image “A” may not be an X-ray image that includes a sufficient amount of internal information of the subject 1000 and is suitable for observation. Moreover, satisfying the requirement of a very narrow dynamic range of the X-ray sensor unit 1802 is first prioritized in determining the X-ray irradiation conditions. Therefore, reducing the X-ray dose that may be absorbed by the subject may not be considered sufficiently.
FIG. 19 illustrates an example of the configuration of a conventional X-ray imaging apparatus 1900. The conventional X-ray imaging apparatus 1900 includes an image processing unit 1901, although the rest of the configuration is similar to that of the X-ray imaging apparatus 1800. In FIG. 19, constituent components and portions similar to those illustrated in FIG. 18 are denoted by the same reference numerals.
In the conventional X-ray imaging apparatus 1900 illustrated in FIG. 19, the image processing unit 1901 performs image processing on the X-ray image “A” that has been generated by the X-ray sensor unit 1802. Namely, the image processing unit 1901 generates an X-ray image “B” based on the X-ray image “A.” Therefore, the display unit 1804 displays the X-ray image “B” that has been subjected to the image processing.
In this case, the X-ray image “B” is an image generated through the image processing applied to the X-ray image “A.” Therefore, the X-ray image “B” may be an adequate X-ray image compared to the image obtained by the apparatus illustrated in FIG. 18. However, even in this case, the X-ray image “A” (i.e., an original image) is an image that has been obtained so as to satisfy the requirement of a very narrow dynamic range of the X-ray sensor unit 1802. Therefore, the X-ray image “B” may not be an X-ray image that is suitable for observation. Moreover, as satisfying the requirement of a very narrow dynamic range of the X-ray sensor unit 1802 is first prioritized in determining the X-ray irradiation conditions, reducing the X-ray dose that may be absorbed by the subject may not be considered sufficiently.